This invention relates generally to a continuously variable transmission (CVT). More particularly, it relates to a CVT adapted for use in a small, front-wheel-drive automotive vehicle. The CVT has a plurality of drive ranges and a synchronous shift point at which it may be shifted from any drive range to any other drive range.
The typical automotive transmission is shifted in descrete steps between a low-speed, high-torque range for starting a vehicle and a high-speed, low-torque range for vehicle operation at highway speeds. In a manual transmission shifting is accomplished by the engagements of gear sets. In an automatic transmission shifting is accomplished by the controlled engagement of friction elements. Because such shifting is in step functions, the most efficient vehicle operation can only be approximated. Automotive engineers have long recognized that efficiency would be improved if the transmission could be adjusted continuously to compensate for changing loads, speeds, etc. This would allow the engine to be operated at maximum efficiency under changing conditions.
CVTs have been known for some time. The typical CVT employs a variable-ratio drive mechanism (variator) including a variable pulley having a pair of flanges mounted on a drive shaft, with one of the flanges being movable with respect to the other so as to change the pulley pitch radius. Another, similar variable pulley is mounted on a driven shaft. A suitable belt or chain couples the pulleys for transferring torque therebetween. When the pitch radius of one pulley is changed, the pitch radius of the other pulley is changed simultaneously in the opposite direction. As a result, the drive ratio between the drive and driven shafts is varied in a continuous, smooth manner within limits defining the ratio range of the variator. Typically, although not necessarily, this range is from a reduction ratio to its inverse overdrive ratio.
In recent years considerable work has been directed to the application of a CVT to the driveline of an automotive vehicle. Examples of such CVTs are disclosed in U.S. Pat. No. 2,933,952 issued Apr. 26, 1960 and U.S. Pat. No. 3,479,908 issued Nov. 25, 1969. Each of these patents discloses a split-path CVT having a low range providing low-speed, high-torque drive in both the forward and reverse directions, but only one high range providing high-speed, low-torque drive in the forward direction. U.S. Pat. No. 3,340,749 issued Sept. 12, 1967 discloses a split-path CVT providing a plurality of high ranges. However, this patent does not disclose a synchronous shift point at which the transmission may be shifted from any one of the drive ranges to any other. Rather, the drive ranges must be engaged sequentially in order to provide a smooth flow of power.
There remains a need in the art for a CVT which allows a vehicle operator to shift from any drive range directly to any other drive range. In particular, it would desirable for the operator to have the option of shifting from a low range either to a first, performance high range suitable for city driving or to a second, economy high range suitable for highway driving.